What's the Hi/Lo algorithm in NHibernate?

A primary key is made up of two numbers - a "high" number and a "low" number.We can basically increment the "high" sequence, knowing that it can then safely generate keys from the entire range of the previous "high" value with the variety of "low" values. E.g. we have a "high" sequence with a current value of 44, and the "low" number is in the range 0-500. Then we can increment the sequence to 45 and know that keys 44/0, 44/1,.... 44/500 are all available.